The present invention relates to a novel primer composition or, more particularly, to a primer composition suitable for priming the surface of an article of a plastic resin such as a polycarbonate resin as a pretreatment of coating with a silicone-based coating composition to form a wear-resistant coating film on the surface. The invention also relates to a method for providing a protective hard coating layer on a plastic-made article by using the above mentioned novel primer composition as well as a thus coated plastic-made article.
As is well known, polycarbonate resins as a class of thermoplastic synthetic resins have excellent transparency and high impact strength along with the advantageously low density so that they are widely used as a structural material in place of inorganic glassy materials. Polycarbonate resins, however, have several disadvantageous properties such as the relatively low wear resistance, susceptibility to the attack of various organic solvents and discoloration or degradation sometimes taking place in the lapse of time.
It is proposed accordingly to provide the surface of a shaped article of a polycarbonate resin with a protective coating film of a thermosetting resin. Among the thermosetting resins hitherto proposed for the purpose, organopolysiloxanes are highlighted in respect of the high wear resistance and solvent resistance. On the contrary to these advantages, silicone-based coating films have defects in respect of the relatively low durability of the coating films due to the poor adhesion to the surface of a thermoplastic resin such as polycarbonate resins and low weatherability or resistance against weathering in the lapse of time. Attempts have been made to solve these problems by using an organopolysiloxane-based coating composition in combination with a primer or undercoating composition comprising another synthetic polymer including (1) a thermoplastic acrylic resin disclosed in Japanese Patent Kokai 52-138565, (2) an acrylic resin having functional groups such as amino groups disclosed in Japanese Patent Kokai 53-138476, (3) a modified organopolysiloxane which is a reaction product of an alkoxy silane having a functional group such as amino and hydroxy and a cyclic acid anhydride compound disclosed in Japanese Patent Kokai 53-81533, (4) a combination of an amino silane and a hydrolysis product of an epoxy silane disclosed in Japanese Patent Kokai 54-63176, (5) a combination of a thermoplastic acrylic resin having functional groups and an ultraviolet absorber disclosed in Japanese Patent Kohyou 55-500809, ( 6) a combination of a thermosetting acrylic resin in the form of an aqueous emulsion and an ultraviolet-absorbing compound disclosed in Japanese Patent Kokai 55-160033, (7) a modified silicone which is a reaction product of an aminosilane, an epoxy silane and an acid anhydride disclosed in Japanese Patent Kokai 56-16573, (8) a modified thermoplastic acrylic polymer which is a reaction product of an acrylic monomer, an epoxy methacrylate and a hydroxybenzophenone-based ultraviolet absorber disclosed in Japanese Patent Kokai 57-23661, (9) an organopolysiloxane containing an alkoxysilyl-functional aromatic ultraviolet absorber and colloidal silica and so on.
Each of these prior art priming or undercoating compositions, however, is not quite satisfactory in one or more respects. For example, the priming treatment by using the above described first or second primer composition is considerably effective in improving the adhesion of the protective coating layer to the substrate surface but substantially no improvement can be obtained thereby in the weatherability of the coating layer rather with a decrease in the resistance against water as a consequence of the chemical structure having active hydrogen atom-containing groups such as amino groups and the like. The third and fourth primer compositions, though effective in improving the adhesion of the coating layer to the substrate surface, have problems of insufficient resistance of bonding against water and decrease in the weatherability due to the presence of active hydrogen atoms in addition to the defect of possible yellowing. Moreover, the primer compositions of this type are subject to an increase in the consistency during storage as a result of the reaction between the hydroxy groups and carboxyl groups and the alkoxy groups in the components so as to decrease the workability in coating and decrease in the priming effect due to the increase in the degree of polymerization. The primer composition of the fifth type is imparted with weatherability improved to a considerable extent in so far as the desired weatherability is for a short time but the weatharability is still insufficient for a long period of time. When the coating thickness thereof is increased with an object to improve the weatherability, a drawback is caused that the hardness of the anti-wearing protecting overcoating layer of the organopolysiloxane composition may eventually be decreased to be less wear-resistant. When the amount of the ultraviolet absorber is increased with the same object, the coating layer may be subject to whitening along with a decrease in the priming effect. The primer composition of the sixth type, which is in the form of an aqueous emulsion, is under a limitation in the types of the ultraviolet absorbers contained therein in respect of the solubility in water so that the weatherability, in particular, under ultraviolet light of the coating layer is improved sometimes only insufficiently. Moreover, it is a defect inherent in thermosetting resins such as acrylic resins that the priming effect for adhesion improvement cannot be high enough. The compositions of the seventh type have similar disadvantages to those in the compositions of the third and fourth types. The compositions of the eighth type cannot contain a sufficiently large amount of ultraviolet absorbers since most of the ultraviolet-absorbing compounds act also as a free radical scavenger to retard the polymerization reaction so that sufficiently high weatherability and priming effect can hardly be obtained. The undercoating compositions of the ninth type is inferior in the adhesiveness through with an improvement in the weatherability.